Ch_322b_14.04

Ch_322b_14.04 - from H-H repulsion Planar Structures of...

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The Annulenes Annulenes are monocyclic compounds with alternating double and single bonds. The ring size is given by a number inside square brackets before the name "annulene." benzene cyclooctatetraene [6]annulene [8]annulene Prediction of Aromaticity of the Annulenes The Huckel (4n+2) rule predicts whether an annulene is expected to show aromatic properties. Over the past 40 years, many annulenes have been synthesized and studied.
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Predictions of Aromaticity by the Huckel Rule: 4n + 2 : Prediction Yes Yes Yes Prediction No Observation Observation No Yes No Yes [10]annulene [14]annulene [16]annulene [18]annulene
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The Trouble with [10]Annulene [10]Annulene does not show the properties and stability of an aromatic compound despite the prediction by the Huckel rule. The required planar structures (with extensive overlap of the p-orbitals) in both the all- cis (A) and naphthalene-like (B) forms are unstable because of strain energy . : A (all cis ) severe angle strain H H B (naphthalene-like) severe steric strain
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Unformatted text preview: from H-H repulsion Planar Structures of [10]Annulene [10]Annulene has a non-planar structure that prevents the continuous overlap of the p-orbitals that is required for an aromatic system. [4]Annulene (1,3-Cyclobutadiene) For many years before its synthesis in 1965, this nonaromatic compound was the focus of great theoretical interest and speculation. R.Petit and coworkers first synthesized cyclobutadiene in 1965 followed by O.L. Chapman and coworkers in 1973. In the latter study, the highly reactive cyclobutadiene was trapped at low temperature following a photochemical synthesis in an argon matrix at -200 o C. O O = pyranone h !-200 o C in argon matrix O unstable O h ! + CO 2 in argon matrix at -200 o C cyclobutadiene dimerizes during warm up Spectroscopic studies on the matrix trapped 1,3-cyclobutadiene confirm that it is not aromatic. It has a rectangular geometry indicating nonequivalent single and double bonds....
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Ch_322b_14.04 - from H-H repulsion Planar Structures of...

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